The present invention relates to an apparatus and a method of measuring the magnetic field distribution of a magnetic sample. More particularly though not exclusively, the present invention relates to a magnetic field distribution measuring system employing a magnetic field scanner together with relatively fast data acquisition and data analysing to provide the user with detailed magnetic field maps in seconds.
In trying to determine the magnetic field distribution in space due to systems containing magnetised ferromagnetic material and/or electromagnetic components, it is very difficult to achieve a precise analytical solution such as may be possible in an electrical counterpart for anything other than the simplest of geometries due to the relative permeability of iron being very much less than the electrical conductivity of copper relative to air.
One advance which has revolutionised the design of electromagnetic systems over the last 15 years is the computer modelling of magnetic fields, the methods most usually employed being finite element, or finite difference, analysis. From the electromagnetic characteristics of the components of a particular system, numerical methods are used to calculate the magnetic field at any given point, calculating the fields at enough points to a high enough resolution giving an effective field distribution. It is now possible to calculate quite complex transient field distributions from sophisticated designs in a matter of minutes on a modem desktop PC.
As with any computer model, however, these methods rely on accurate information regarding the behaviour of the individual components in varying magnetic fields, particularly ferromagnetic materials such as permanent magnets or soft magnetic iron alloys.
This requires very accurate materials characterisation to determine the relevant magnetic parameters, and also assumes consistent material properties across one or more supplied batches, which is often difficult to guarantee.
Permanent magnets, in particular, are very difficult to characterise completely due to the innate hysteretic nature of their behaviour.
Thus, despite the great advances in design made possible by electromagnetic computer modeling, there exists a great need for actual physical measurement of magnetic field distributions.
Conventional measurement systems lag far behind the representations and visualisations afforded by computer modelling, however, offering, at best, measurement of varying fields at a point, rather than the field distribution over an area.
Accordingly, it is an object of the present invention to provide a magnetic field distribution measurement system which overcomes at least some of the above problems.
According to one aspect of the present invention there is provided an apparatus for measuring a magnetic field distribution of a magnetic sample, the apparatus comprising magnetic field sensing means for sensing the magnetic field produced by the magnetic sample at a given location; scanning means arranged to move said sensing means along a predetermined scan pattern; and data acquisition means for sampling data from said sensing means, said sampled data providing a representation of the magnetic field distribution of the magnetic sample.
The data acquisition means is preferably arranged to carry out relatively fast data acquisition and to store the sampled data in a storage means in real time. In one embodiment of the invention, a typical scan pattern covering an area of 20 mmxc3x9720 mm at a resolution of 0.1 mm (i.e. 40,000 data points) takes less than 90 seconds to complete. In this embodiment, the magnetic field sensing means and the data acquisition means are arranged to sense the magnetic field in three orthogonal dimensions at each sampled location (data point) which helps to establish the magnitude and direction of the magnetic field at that location.
The relatively high speed of operation of the data acquisition means can be achieved by using a dedicated processor means and a storage means as part of the acquisition means. If the acquisition means is formed as a slot-in card for a Personal Computer (PC), then data can be sampled, stored in temporary memory on the card and transferred to the main storage means (memory) of the PC using direct memory access, for example.
The data acquisition means is preferably arranged to control movement of the scanning means. This advantageously ensures that the coordination between the sampling of the sensing means and its scanning movement can both be synchronised. In addition, the status of the scanning means is preferably checked by the data acquisition means to ensure that the apparatus is only operated under predetermined status conditions.
The apparatus preferably includes processing means for processing the sampled data into an image format and for displaying the data on display means, such as on a high resolution colour monitor as a coloured magnetic field map. The processing means may also include means for analysing the sampled data to determine magnetic force vectors or lines of force which can be superimposed on the displayed magnetic field map. In addition, magnetic field strength cross-sectional information can also preferably be provided by the analysing means. This display of magnetic field data allows easy comparison with computer modelled predictions of magnetic field, providing an invaluable check with the xe2x80x9creal worldxe2x80x9d for computer modellers.
According to another aspect of the present invention there is provided a scanning apparatus for scanning a magnetic field of a magnetic sample, said apparatus comprising: a scanning head comprising at least one magnetic field sensor; first moving means for controllably moving the scanning head in a first coordinate dimension; and second moving means for controllably moving the scanning head in another coordinate dimension, said first and second moving means being controllable to effect scanning of said head over a plane surface above the magnetic sample.
According to another aspect of the present invention there is provided a magnetic field scanning head for use with a scanning apparatus, said head comprising a plurality of magnetic field sensors spaced apart along a scan line, each sensor sensing magnetic field strength in a different direction to the other sensors, and the head being arranged to be moved along said scan line during a scanning operation such that each sensor can sense the magnetic field in its direction at a given location.
According to another aspect of the present invention there is provided a method of measuring a magnetic field of a magnetic sample, the method comprising: sensing the magnetic field produced by the sample at a given location using sensing means; moving the sensing means to a new location; repeating the sensing and moving steps to effect a scan of the magnetic field distribution over a predetermined scan area; and sampling the magnetic field data produced by the sensing means, said sampled data providing a representation of the magnetic field distribution of the magnetic sample.
The present invention has application in a method of carrying out quality control checks on a production line of magnetic samples and in computer aided engineering of permanent magnets, magnetic components and/or magnetising fixtures.
The above and further features of the present invention are set forth with particularity in the appended claims and will become clearer from consideration of the following detailed description of exemplary embodiments of the present invention given with reference to the accompanying drawings.